Quite simply: 2+x
A constant is simply a number, something that does not change: here it's "2"
A variable can chage: here it's "x" - anything can be substituted for x
If we put those two together with a "+" sign, we get an expression: variables or contants (including multipled ones: such as 2x, which is a term) added to each other or subtracted from each other
